A compressor comprises an impeller wheel, having a plurality of blades (or vanes) mounted on a shaft for rotation within a compressor housing. In the case of a centrifugal compressor, rotation of the impeller wheel causes gas (e.g. air) to be drawn into the impeller wheel and delivered to an outlet volute defined, at least in part, by the compressor housing around the impeller wheel.
One use of a compressor is in a turbocharger. Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises a housing in which is provided an exhaust gas driven turbine wheel mounted on a rotatable shaft connected downstream of an engine outlet manifold. A compressor impeller wheel is mounted on the opposite end of the shaft such that rotation of the turbine wheel drives rotation of the impeller wheel. In this application of a compressor, the impeller wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems.
A known centrifugal compressor housing comprises an axial intake, an annular diffuser and an annular outlet volute in the form of a scroll volute. An impeller, with a plurality of blades, is mounted on a shaft, for rotation about a longitudinal axis of the compressor housing, and is received between the axial intake and the outlet volute.
A radially inner surface of the axial intake forms an annular intake passage that extends axially inboard from an intake port to the impeller wheel.
The diffuser comprises first and second wall members having respectively opposed first and second surfaces that define an annular diffuser passage that surrounds the impeller and extends in a radially outward direction from an annular diffuser inlet downstream of said plurality of blades, the tips of the blades sweeping across said diffuser inlet during use, to an annular diffuser outlet communicating with the annular outlet volute. The diffuser outlet is formed by respective annular outlet ends of the first and second surfaces.
An inner surface of the outlet volute defines an annular outlet volute passage that extends, along a circumferentially extending volute passage axis, about the compressor housing longitudinal axis.
In use, as the impeller rotates, air is drawn in from the intake port, through the axial intake, to the impeller and passes from the impeller through the diffuser passage to the annular outlet volute passage. The compressed air passes along the outlet volute passage and out through a volute outlet to a desired location, e.g. to an engine intake manifold.
The inner surface of the volute extends, in a circumferential direction about the volute passage axis, from the annular outlet end of the first surface that defines the diffuser passage to the annular outlet end of the second surface that defines the diffuser passage. The inner surface has a generally constant radius, relative to the volute passage axis, such that the inner surface of the volute has a generally circular cross-sectional shape about the volute passage axis.
The inner surface of the volute has an annular first section that extends axially outboard (i.e. away from the diffuser passage) from the annular outlet end of the first surface that defines the diffuser passage.
It is known to form the first section of the inner surface of the volute such that it extends radially inwardly (relative to the compressor housing longitudinal axis) of the annular outlet end of the first surface that defines the diffuser passage to form a radially outwardly protruding annular lip, curved along its radial extent, that extends along the annular inlet end of the first surface. Providing this curved lip is advantageous in that it acts to better align the circulating flow in the outlet volute, as it passes from the first section of the inner surface of the volute towards the diffuser outlet, with the flow leaving the diffuser outlet, thereby reducing losses. The shape of the first section to form the lip is produced by appropriate shaping of the outer surface of a core around which the compressor housing is cast (for example a sand core or metal core, as described below).
An outlet volute may be formed from a single piece or from multiple pieces that are subsequently attached together.
It is known to use sand casting to produce a single piece closed volute with a cross sectional shape having this lip. In sand casting, a die is located around a sand core. A suitable bonding agent (usually clay) is typically mixed with the sand and the mixture is moistened, typically with water, but sometimes with other substances, to provide the strength and plasticity of the core suitable for moulding. The sand is compacted around a mould to provide the required shape of the core.
The die is positioned to enclose the sand core to define a mould cavity between an inner surface of the die and an outer surface of the sand core. Accordingly, an inner surface of the die defines the shape of the outer surface of the outlet volute (as well as of the diffuser and axial intake) and an outer surface of the sand core defines the shape of the inner surface of the outlet volute (as well as of the diffuser and axial intake).
Molten metal is injected into the mould cavity. Once the molten metal cools and solidifies, the die is removed and the sand core is removed from the inside of the compressor housing by tipping the sand particles out through the volute outlet.
Sand casting is disadvantageous in that, during the casting process, the shape of the sand core can change, resulting in dimensional inconsistency. In addition, it produces a relatively poor surface finish which, during use, results in losses in the flow.
It is also known to use pressure die casting to produce a multiple piece closed volute with this cross sectional shape. In pressure die casting molten metal is forced under pressure into a mould cavity. The mould cavity is defined between an inner surface of a die and an outer surface of a metal core located within the die.
In this process, multiple sections of the compressor housing (axially opposed sections) are formed separately, using pressure die casting, and are then assembled together to form a volute inner surface with the above cross sectional shape (a circular cross-sectional shape provided with said lip). Pressure die casting is advantageous in that it provides a better surface finish than sand casting, which gives better performance and reduces losses in the flow. However, due to the interfaces between the multiple sections, the volute has problems of leakage and containment issues, resulting in losses and inefficiencies in the flow.
Furthermore, it is currently not possible to use pressure die casting to form a single piece volute having a cross sectional shape provided with said lip, since the lip would prevent the metal core from being removed out of the volute after the casting process is complete.
In addition, due to the relatively high tooling costs with pressure die casting, it is necessary for high volumes of the compressor housing to be manufactured in order for the manufacturing process to be economically viable.
It is an object of the present disclosure to obviate or mitigate one or more of the problems set out above. A further object of the present disclosure is to provide an alternative method of manufacturing a compressor housing, compressor and turbocharger. A yet further object of the present disclosure is to provide a compressor housing, compressor and turbocharger manufactured according to the alternative method.